There is known a coin telephone, and more particularly a combination pay and general subscriber telephone set from U.S. Pat. No. 3,511,939, which is capable of operating either as a pay station, or upon insertion and operation of a key, as a general subscriber set.
A combination pay and general subscriber telephone service is provided, which has sensing means operatively associated with the dial assembly to sense dialing of a prohibited digit in a particular sequential place within a telephone number. Additionally the set is provided with key operative means which are connected to the circuit and, upon operation of the key, switch the circuit to a regular subscriber telephone circuit, and prevent disabling of the connection of the set to the telephone line, regardless of the number being dialed. A dial has a cam associated therewith which is positioned to have a predetermined relation with respect to the position of the prohibited digit on the dial.
The U.S. Pat. No. 3,727,009, there is also known a telephone security device which is intended to prevent unauthorized use of the distant-direct-dialing facility on dial-type telephones. It makes use of a mechanism which is stepped by operation of the dial to call a digit, and which after a predetermined number of digits has been called, locks the dialing mechanism to prevent further dialing. The predetermined number is so selected, as to prevent the desired misuse of the telephone.
From U.S. Pat. No. 3,832,497 there is also known a telephone long distance attachment to be attached to a standard telephone on the outside housing thereof, without in any way dismantling the telephone, for preventing long distance calls, and at the same time facilitating local calls in which a first ratchet mechanism limits the number of dialed digits to those required for a local call only. A second ratchet mechanism prevents dialing the operator on the first dial, and a third ratchet mechanism prevents the release of the telephone buttons before they are fully pressed, to prevent dialing by depressing the buttons.
From U.S. Pat. No. 3,748,402 there is also known an apparatus for preventing the dialing of toll call telephone numbers using a public utility rotary dial telephone. The apparatus permits local telephone calls to be completed, and can be disabled to permit toll calls by a local call pay telephone.
There is also known from U.S. Pat. No. 4,022,992, a telephone lock for attachment to a telephone which permits the user to dial local calls only with a first predetermined number of digits, where one of the digits is assigned to call an operator. In this telephone lock, which has been invented by the inventor of the present invention, the telephone receiver normally rests upon a set of depressable push-buttons for making contact with the push-buttons of the telephone itself. A lock dial device is connected to the telephone dialing device, so that the user can dial a selected number of the digits sequentially when the telephone receiver and speaker have been lifted from the set of push-buttons. First and second dialing stop-devices prevent the user from dialing the digit assigned to the operator, and from dialing an additional number of digits after he has dialed the first number of digits, respectively. A counting device for counting the first number of digits, includes a shaft, a gear rotatable on the latter, and formed with a plurality of substantially equispaced longitudinal fingers; one of the fingers is an elongated finger. The torsion spring urges the gear to move in one direction, and the gear may be advanced an angular step equal to the spacing between adjacent fingers, when dialing one of the digits. The second dialing stop-device stops the advancement of the gear, when a bar attached to the lock's housing impinges on the elongated finger. When the receiver/speaker has been replaced on the set of push-buttons, the dialing device is reset to an initial state.
All of the above inventions are relatively complicated devices, which have a relatively large number of parts, and cannot easily be manufactured at low cost.